I know it's way over budget and you don't want a trailer, but I can't believe it's taken so long for companies to start producing these:
Yakima Rack and Roll Trailer - 66" Yakima Trailers Y08106
Yakima = way overpriced ($1,719.20) but I LOVE the concept. Super skinny tires for LRR and aero, extremely lightweight (150lbs for the trailer) and carries a 250lb box with standard roof rack mounts, but nice and low for easy access... and tucked nicely into your slipstream. On fairly flat land, there'd probably be almost no hit at all to MPG (obviously more on hilly terrain). Just posting the fact that they exist for more people to know something so awesome exists
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It's my fantasy road trip rig for sure.